Suppose I want to do something like:

If I want, I could build a list of arguments up on the fly, and pass the list to the command to be run, and everything would be funky.

    my @args = map{ ($_, foo($arg{$_})) } keys %args;
    my @output = `/bin/foo @args`;
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I have a couple of problems with this, in that the @args variable doesn't really serve any purpose, apart from allowing me to avoid the rather cumbersome:

But in any event it just so happens that foo() can return strings containing $ characters, which are then munged by the shell, and the called program gets confused. I can't use the single quoted variant of qx//, because then @args won't be interpolated, which is even worse.

In Programming Perl 3^rd Edition, it explains that "backticks don't have a list-argument form that is proof against the shell," but it goes on to say "a multi-argument form of the underlying readpipe operator is in development; but as of this writing, it isn't quite ready for prime time".

In terms of system, you can choose, or not, to pass through the underlying shell by doing:

  system( '/bin/foo    -p    stuff' ); # passes by the shell
  system( '/bin/foo', '-p', 'stuff' ); # bypasses the shell
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Backticked commands are always fed to the shell. This poses a problem if the command line in question contains a dollar sign, because the shell will try and interpolate an evironment variable in its place and all hell breaks loose.

That is, there is no syntax to do:

  my @out = `/bin/foo`, qw/-p stuff/;
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You have to go the whole hog, and fork and read back from your child. Which gives something like:

my $pid;
if( !defined($pid = open( KIDP, '-|' ))) {
    print child fork error: $!\n";
}
elsif( $pid ) {
    while( <KID> ) {
        chomp;
        # digest $_
    }
    close KID;
}
else {
     $ENV{PATH} = '/bin:/usr/bin:/usr/local/bin';
     my @args = map{ (foo, foo($_)) } keys %data;
     exec '/usr/local/bin/check-netscreen', @args;
     die "ping child died \$!=$! \$@=$@\nargs = @args";
}
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#! /usr/bin/perl -w

use strict;

my @out = readpipe( '/bin/foo', qw/one two three/ );
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Can't exec "three": No such file or directory